The present invention generally relates to ultrasound imaging of tissue structures. In particular, the invention relates to a method and an apparatus for improving the display of ultrasound imaging of tissue structures.
For a number of years, ultrasound imaging has been used to non-invasively monitor and image tissue structures within the human body. To produce the image, an ultrasonic transducer transmits an ultrasonic wave of energy into a patient's body. When the ultrasonic wave encounters a tissue interface, the difference in impedance between the two tissues at the interface causes an ultrasonic echo to be reflected back at the ultrasonic transducer. The time required for the ultrasonic echo to return to the ultrasonic transducer is used to determine the location where the ultrasonic echo originated. By determining where the echo originated, the location of the tissue interface causing the ultrasonic echo may be located within the patient.
As the ultrasonic wave impinges upon a tissue interface, the amplitude of the resulting ultrasonic echo depends upon the type of tissues on both sides of the interface. Some tissues will create an ultrasonic echo with a higher amplitude than other tissues. For example, an ultrasonic echo produced at a transition region from fat to muscle will generate an ultrasonic echo with a different amplitude than a transition region from a blood vessel wall to blood.
A B-mode image is an image that uses the differences in amplitude between the ultrasonic echoes to represent the tissue structure of a patient. Higher amplitude ultrasonic echoes are typically represented on the image as brighter spots and lower amplitude ultrasonic echoes are typically represented on the image as dimmer spots. By plotting the spots of varying brightness on a display, the resulting display provides an image of the transition regions between tissue structures in the patient. Because the transition regions outline the tissue structure, medical personnel are able to non-invasively see the organ and tissue structure of the patient.
Similar to a B-mode image, a color power Doppler image focuses upon the amplitude of the received ultrasonic echoes. However, unlike a B-mode image, the color power Doppler image only depicts the amplitude of ultrasonic echoes that exhibit a Doppler shift in frequency. As explained in more detail below, by only depicting the ultrasonic echoes that exhibit a Doppler shift in frequency, the color power Doppler image only depicts tissue structure that is moving inside the patient.
To produce a color power Doppler image, the ultrasonic transducer produces and transmits ultrasonic waves of energy into a patient. The ultrasonic waves travel to various depths in the body until impinging upon tissue interfaces in the body that reflect ultrasonic echoes of the ultrasonic waves back towards the transducer array.
If an ultrasonic wave impinges upon a tissue interface that is moving, the ultrasonic echo reflected back at the transducer array will contain a different frequency than the impinging ultrasonic wave. For example, if the ultrasonic wave impinges upon a tissue interface moving towards the ultrasonic transducer, the resulting ultrasonic echo will have a higher frequency than the original impinging ultrasonic wave. On the other hand, if the ultrasonic wave impinges upon a tissue interface moving away from the ultrasonic transducer, the resulting ultrasonic echo will have a lower frequency than the original impinging ultrasonic wave.
The difference between the frequency of the impinging ultrasonic wave and the resulting ultrasonic echo is referred to as a phase shift. To produce a color power Doppler image, only the amplitudes of ultrasonic echoes containing phase shifts from corresponding ultrasonic waves are displayed. Higher amplitude ultrasonic echoes are typically represented on the image as brighter spots and lower amplitude ultrasonic echoes are typically represented on the image as dimmer spots. By plotting the spots of varying brightness on a display, the resulting display provides an image of the transition regions between moving tissue structures in the patient. Consequently, medical personnel are able to non-invasively see the moving organ and tissue structure within the patient.
Similar to a color power Doppler image, a color velocity Doppler image focuses upon ultrasonic echoes that exhibit a phase shift in frequency. However, unlike a color power Doppler image, a color velocity Doppler image depicts the velocity at which tissue is moving within a patient's body.
In color velocity Doppler imaging, the level of phase shift in ultrasonic echoes is translated into a velocity value. For example, tissue moving towards the transducer array produces ultrasonic echoes with a higher frequency than the impinging ultrasonic wave. The higher the frequency of the ultrasonic echo, the higher the velocity of the movement of the tissue towards the transducer array. In contrast, tissue moving away from the transducer array produces ultrasonic echoes with a lower frequency than the impinging ultrasonic wave. The lower the frequency of the ultrasonic echo, the lower the velocity of movement of the tissue away from the transducer array.
While the intensity or amplitude of an ultrasonic echo depends on the tissue interface from which the ultrasonic echo originates, the intensity of the ultrasonic echo also depends on how deep within the patient the tissue interface is located. The deeper within the patient the tissue interface is located, the more the amplitude of the ultrasonic echo will be attenuated before it is received by the ultrasonic transducer. The more the ultrasonic echo is attenuated, the weaker the ultrasonic echo and the dimmer the spot that graphically represents the ultrasonic echo in the image. Consequently, a B-mode image and color power Doppler image gradually darken as the location on the image gets farther from the location of the ultrasonic transducer.
A conventional way to compensate for attenuation of ultrasonic echoes in ultrasound systems has been to manually adjust gain levels by rotating knobs. An operator of an ultrasound system typically selects a region of interest on a display monitor and rotates a gain knob to adjust intensity level of the displayed image to better depict the region of interest on the display. For example, if the region of interest is too dim, then the operator will increase the gain by rotating the gain knob in a positive direction. Unfortunately, increasing the gain in a region of interest to a desirable level may increase the gain in other regions on the display to undesirable levels. By increasing the gain in regions representing less attenuated data, the quantity of noise displayed in the regions representing less attenuated data may be increased. Consequently, a need exists for an ultrasound system that automatically adjusts gain level and equalizes gain level as a function of depth.
In addition, if the overall gain is too low in comparison to the noise floor, an automatic gain system that equalizes gain as a function of depth may overcompensate for the low level of overall gain. The overcompensation may produce an image with too much digital gain from the automatic gain system and not enough analog gain from a front end gain system that provides overall gain. Consequently, a need exists for a system that automatically adjusts overall gain of a front end gain system to be within a few decibels of the noise floor.
In addition, the amount of gain applied to compensate for attenuation of ultrasonic echoes and equalize gain as a function of depth may change depending on the type of wall filter used. The type of wall filter used radically effects the appearance of noise in the displayed image. Thus, the type of wall filter used will effect the noise floor. Consequently, a need exists for a system that automatically adjusts gain level, equalizes gain as a function of depth, and compensates for the type of wall filter used.